The present invention relates to esters of 3-tert-butyl-and 3-tert-butyl-5-alkyl-4-hydroxyphenyl (alkane) carboxylic acids, their preparation and their use as polymer stabilizers.
It is known that organic polymers as obtained by polymerization (or copolymerization) of mono- and diolefins, optionally containing functional groups, or by polycondensation of suitable precursors--for example, of diols with dicarboxylic acids--can suffer changes under the effect of air/oxygen, heat, light, or high-energy radiation; these changes impair the properties of the polymers important in their practical usage, such as strength, hardness and elongation. On account of such impairments, there is frequently not only a marked change in measurable physical properties, but also a visually noticable softening, brittleness and/or discoloration of the finished articles also occur. For this reason, stabilizers are added to the corresponding polymers prior to processing. For more details, see the review publications by G. Scott, "Atmospheric Oxidation and Antioxidants", Elsevier Publ. Co. Amsterdam, Oxford, N.Y. (1965); R. Gachter, H. Muller, "Taschenbuch der Kunststoff-Additive"[Pocket Manual of Plastics Additives], C. Hanser Publishers, Munich/Vienna (1979); J. Pospisil in "Degradation and Stabilization of Polymers" (Edit.: H. Jellinek), Elsevier, Amsterdam, Oxford, N.Y. (1983), pp. 193 et seq.; P. P. Klemchuk et al., "Polymer Degradation and Stabilization" 7: 131 et seq. (1984).
It is also known that in case of polyolefins, compounds containing phenolic hydroxy groups are preferably utilized as stabilizers, and that, among these phenol derivatives, materials having voluminous alkyl groups, preferably with tert-butyl substituents, in at least one ortho position with respect to the phenolic hydroxy group, exhibit a particularly high efficacy. Esters can also be found amoung the large number of stabilizers disclosed (compare, for example, J. C. Johnson, "Antioxidants", Noyes Data Corp., 1975; M. W. Ramsey, "Antioxidants-Recent Developments", Noyes Data Corp., 1979; M. T. Gillies, "Stabilizers for Synthetic Resins", Noyes Data Corp., Park Ridge, N.J., 1983). Esters of 3,5-dialkyl-4-hydroxyphenyl(alkane) carboxylic acids are described, for example, in U.S. Pat. Nos. 3,681,431; 3,330,859; 3,644,482; 3,285,855; 4,598,113; esters of 4,6-dialkyl-3-hydroxyphenyl(alkane) carboxylic acids are disclosed in U.S. Pat. Nos. 3,988,363; 3,862,130; and esters of 2-methyl-4-tert-butyl-5-hydroxyphenylalkane carboxylic acids are set forth in European Pat. No. 0.048,841.
The efficacy of the phenolic stabilizers can frequently be increased by adding specific compounds which in most cases contain sulfur or phosphorus. The optimum quantitative ratio of stabilizer and synergist is always to be found empirically in each individual instance.
The stabilizers as well as the synergists must meet the criteria that they can be incorporated without difficulties into the various polymers without decomposing, and that they can be distributed therein with maximum uniformity. On the other hand, the polymer should, at the required high incorporation temperatures, neither be discolored by additives nor suffer molecular weight degradation due to temperature load and shear stress.
In the literature, there are various stabilizer compounds disclosed which contain, besides sterically protected hydroxy phenyl groups, sulfur atoms which function synergistically in the same molecule (cf., e.g., R. W. Layer, "Non-staining Antioxidants" (in G. Scott, "Developments in Polymer Stabilization 4 (1981), p. 163 ff, p. 167); F. X. O'Shea in "Advances Chem. Series" 85 (1968), p. 128 ff; G. Scott in "Developments in Polym. Stabiliz." 6 (1983), p. 29 ff). In particular, derivatives of ortho-thiobisphenol are disclosed as strongly discoloring, and are probably used only for stabilizing rubber mixtures.